Process for producing electret and production apparatus

ABSTRACT

A process for manufacturing an electret article, comprising passing melt-extruded thermoplastic resin fibers through a mist space substantially formed from droplets of a polar liquid, and then collecting the fibers, wherein the thermoplastic resin fibers contain electrical-chargeability enhancing agents, and the average diameter of the droplets is less than 20 μm, is disclosed. Further, an apparatus for manufacturing an electric article, comprising (1) a means for melt-extruding a thermoplastic resin containing electrical-chargeability enhancing agents to form thermoplastic resin fibers; (2) a means for spraying droplets consisting essentially of a polar liquid to a space downstream of a direction of said thermoplastic resin extruded from said means for melt-extruding a thermoplastic resin, to thereby form a mist space, the average diameter of said droplets being less than 20 μm; and (3) a means for collecting said thermoplastic resin fibers which have been passed through said mist space, is also disclosed.

TECHNICAL FIELD

The present invention relates to a process and an apparatus formanufacturing an electret article.

BACKGROUND ART

A nonwoven fabric filter has been used to capture dust in a gas.Nonwoven fabric filters capture dust or the like mainly by physicalfunctions such as Brownian diffusion, an interruption, or an inertialimpaction. Therefore, a nonwoven fabric filter made of finer fibers cancapture and remove finer dust, and thus improve the dust collectionefficiency. Nevertheless, there is a problem in that the pressure lossis increased as the fiber diameter of the nonwoven fabric filter isreduced, and thus, the lifetime of the nonwoven fabric filter isshortened.

As a method for solving the above problem, an attempt to achieve both anenhancement of the collection efficiency and a reduction of the pressureloss was conducted by the electrical charging of the nonwoven fabricfilter to thereby utilize a static electric function in addition to thephysical functions. For example, WO01/27371A1 (Patent Document 1)discloses a method and an apparatus for electrical charging of theindividual fibers by spraying a sufficient amount of polar liquid onto anonconductive polymer before forming a fiber web, collecting the fibersto form a nonwoven web, and then drying the web. According to thismethod and the apparatus, an electret article can be obtained withoutapplying an electric field, by spraying of the polar liquid directlyonto the individual fibers before forming the fiber web and drying theindividual fibers.

The method and the apparatus disclosed in the Patent Document 1 hasadvantageous effects in that a large amount of electrical charge can beacquired, and after the fiber web is formed, a procedure forelectrically charging the resulting fiber web is not required to obtainan electret web. Nevertheless, in the method and the apparatus, thenonconductive polymer must be thoroughly saturated with the polar liquidto be electrically charged, and thus, the drying step is required. Thismeans that a large amount of energy is required for the drying.

The inventors of the present invention engaged in intensive research toovercome the above disadvantages of the prior art and, as a result,found that an electret article having a large amount of electricalcharge can be produced through a simple process without the need of alarge amount of energy for drying.

The present invention is based on the above findings.

DISCLOSURE OF THE INVENTION

The present invention relates to a process for manufacturing an electretarticle, comprising passing melt-extruded thermoplastic resin fibersthrough a mist space substantially formed from droplets of a polarliquid, and then collecting the fibers, wherein the thermoplastic resinfibers contain electrical-chargeability enhancing agents, and theaverage diameter of the droplets is less than 20 μm.

The present inventors found that, according to the process of thepresent invention as above, a large amount of electrical charge can beimparted by passing the extruded thermoplastic resin fibers containingelectrical-chargeability enhancing agents through the mist space formedfrom the droplets of a polar liquid, and a drying procedure becomesunnecessary because an average diameter of the droplets used is lessthan 20 μm, and thus, the thermoplastic resin fibers are not wetted. Itis believed that smaller droplets have a higher surface tension, andthus, do not wet the thermoplastic resin fibers.

Therefore, in a preferred embodiment of the process of the presentinvention, the thermoplastic resin fibers are not subjected to a dryingstep after passing through the mist space.

In another preferred embodiment of the process of the present invention,a resin-droplet percentage of the formula:(Wp/Wf)×100wherein Wp denotes the amount of the droplets forming the mist space andsprayed to a unit volume thereof within a certain period of time, and Wfdenotes the amount of the melt-extruded thermoplastic resin passedthrough the mist space within a certain period of time, is 500 or more.The present inventors also found that, when the resin-droplet percentageis 500 or more, that is, the amount of the droplets in the mist space isrelatively large with respect to the amount of the thermoplastic resin,a large amount of electrical charge can be imparted.

In still another preferred embodiment of the process of the presentinvention, a heated gas is blown onto the melt-extruded thermoplasticresin fibers. In this embodiment, the fibers are drawn and become finer.Thus, an electret article formed from finer fibers, and accordinglyhaving enhanced properties, can be obtained. For example, an electretarticle having a high collection efficiency, an electret article havingan excellent feeling, an electret article having an excellent separatingperformance, or an electret article having an excellent masking propertycan be produced.

In still another preferred embodiment of the process of the presentinvention, a volume specific resistivity of the thermoplastic resin is10¹⁴ Ω·cm or higher, particularly 10¹⁶ Ω·cm or higher. In thisembodiment, the amount of electrical charge can be increased.

In still another preferred embodiment of the process of the presentinvention, the polar liquid is water. In this embodiment, theenvironment wherein the electret article is produced becomes good.

In still another preferred embodiment of the process of the presentinvention, the electrical-chargeability enhancing agent is at least onecompound selected from a group consisting of a hindered amine compound,a metallic salt of a fatty acid, a metallic oxide, and an unsaturatedcarboxylic acid-modified high-molecular compound. In this embodiment,the amount of electrical charge can be remarkably increased.

In still another preferred embodiment of the process of the presentinvention, the average diameter of the droplets is 15 μm or less. Inthis embodiment, the amount of electrical charge can be furtherincreased. This is because the number of droplets can be increased underthe condition of the same total amount of droplets.

The present invention also relates to an apparatus for manufacturing anelectric article, comprising (1) a means for melt-extruding athermoplastic resin containing an electrical-chargeability enhancingagent to form thermoplastic resin fibers; (2) a means for sprayingdroplets consisting essentially of a polar liquid to a space downstreamof a direction of said thermoplastic resin extruded from said means formelt-extruding said thermoplastic resin, to thereby form a mist space,the average diameter of said droplets being less than 20 μm; and (3) ameans for collecting said thermoplastic resin fibers which have beenpassed through said mist space.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of an apparatus for manufacturingan electret article according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The process and the apparatus of the present invention will bedescribed, referring to FIG. 1, i.e., a schematic sectional view of anapparatus for manufacturing an electret article according to the presentinvention.

A melted resin is first extruded from a melt-extruding apparatus 10 inthe form of fibers. The thermoplastic resin is supplied from a vessel(not shown) for supplying the thermoplastic resin to the melt-extrudingapparatus 10, to which electrical-chargeability enhancing agents aresupplied from a vessel (not shown) for supplying theelectrical-chargeability enhancing agent. Thus, the thermoplastic resinfibers 20 contain the electrical-chargeability enhancing agents. Whenthe thermoplastic resin contains the electrical-chargeability enhancingagents, it is not necessary to supply the electrical-chargeabilityenhancing agents to the thermoplastic resin. Alternatively, thethermoplastic resin can be mixed with the electrical-chargeabilityenhancing agents before or after supplied to the melt-extrudingapparatus 10. As above, the thermoplastic resin fibers 20 contain theelectrical-chargeability enhancing agents. Therefore, an electretarticle 60 having a large amount of electrical charge can be produced,according to the present invention. The thermoplastic resin fibers 20are supplied to and passed through a mist space 30 formed from droplets31 of a polar liquid. The thermoplastic resin fibers 20 are electricallycharged when passed through the mist space 30. It is believed that thedroplets 31 of a polar liquid which droplets form the mist space 30 havean average diameter of 20 μm or less and so have a higher surfacetension, and thus, the thermoplastic resin fibers 20 are not wetted.Therefore, it is possible to omit a procedure for drying thethermoplastic resin fibers 20 in the present invention.

The mist space 30 is formed from the droplets 31 sprayed from adroplets-spraying apparatus 40. When the amount of the droplets sprayedis larger than that of the thermoplastic resin fibers 20 extruded, anelectret article 60 having a larger amount of electrical charge can beproduced. The resulting electrically-charged thermoplastic resin fibers21 are collected by a belt conveyer 50 located under the melt-extrudingapparatus 10, whereby the electret article 60 is formed thereon. Theelectret article 60 is not wetted, and thus can be used as an electretproduct without being dried, or can be supplied to subsequent steps forprocessing for other applications without being dried.

The thermoplastic resin which may be used in the present invention isnot particularly limited, so long as it may be extruded. Preferably, thethermoplastic resin is made of a resin having a volume specificresistivity of 10¹⁴ Ω·cm or higher, more preferably 10¹⁶ Ω·cm or higher.Such a resin can result in the increase of the amount of electricalcharge. There is no upper limit of the volume specific resistivity. Theterm “volume specific resistivity” as used herein is a value determinedby a measuring apparatus of a volume specific resistivity, whichapparatus is used in an insulation resistance test by a three-terminalsmethod in accordance with a “Testing method for thermosetting plastics”prescribed in JIS K6911.

Specifically, the thermoplastic resin is, for example, apolyolefin-based resin, such as a polyethylene-based resin,polypropylene-based resin, polymethylpentene-based resin, orpolystyrene-based resin, polytetrafluoroethylene, polyvinylidenechloride, polyvinyl chloride, polyurethane, or the like. In theseresins, the polyolefin-based resin is preferably used, because it has ahigh volume specific resistivity, and has an easy-processing propertybecause of a thermoplasticity thereof. The polypropylene-based resin, orpolymethylpentene-based resin is more preferably used, because it has anexcellent heat resistance.

In the present invention, the amount of electrical charge can beincreased by the electrical-chargeability enhancing agents contained inthe thermoplastic resin. The electrical-chargeability enhancing agent isnot particularly limited, so long as it can increase the amount ofelectrical charge in an article containing itself. More particularly, anelectret article containing a certain compound and an electret articlenot containing the same compound are produced by the process of thepresent invention. When the amount of electrical charge in the electretarticle containing the compound is increased in comparison with that ofelectrical charge in the electret article not containing the compound,the compound can be determined as the electrical-chargeability enhancingagent.

A preferred electrical-chargeability enhancing agent used in the presentinvention is, for example, a hindered amine compound, a metallic salt ofa fatty acid, for example, a metallic salt of a fatty acid having 10 to30 carbon atoms, such as magnesium stearate or aluminum stearate, or thelike, a metallic oxide, such as titanium oxide, silicon dioxide, anaturally-occurring mineral, and an unsaturated carboxylic acid-modifiedhigh-molecular compound, such as polyethylene, polypropylene, orpolystyrene modified by an unsaturated carboxylic acid having 3 to 10carbon atoms, such as acrylic acid, methacrylic acid, or maleic acid.The electrical-chargeability enhancing agent as above can be used alone,or in combination thereof. Of these compounds, the hindered aminecompound can particularly increase the amount of electrical charge, andthus is preferable.

The hindered amine compound is, for example,poly[{(6-(1,1,3,3-tetramethylbutyl)imino-1,3,5-triazine-2,4-diyl){(2,2,6,6-tetramethyl-4-piperidyl)imino}hexamethylene{(2,2,6,6-tetramethyl-4-piperidyl)imino}}],or dimethylsuccinate-1-(2-hydroxyethyl)-4-hydroxy-2,2,6,6-tetramethylpiperidinepolycondensate, or 2-(3,5-di-t-butyl-4-hydroxybenzyl)-2-n-butyl malonicacid bis(1,2,2,6,6-pentamethyl-4-piperidyl).

The amount of the electrical-chargeability enhancing agents contained inthe thermoplastic resin is not particularly limited, but is preferably0.01 to 5 mass % with respect to the total mass of the thermoplasticresin and the electrical-changeability enhancing agent. When the amountof the electrical-chargeability enhancing agent is less than 0.01 mass%, the amount of electrical charge might be insufficient. Thus, theamount is more preferably 0.05 mass % or more, or most preferably 0.1mass % or more. When the amount of the electrical-chargeabilityenhancing agent is more than 5 mass %, a physical strength of theelectret article might be lowered. Thus, the amount is more preferably 4mass % or less, or still more preferably 3 mass % or less, or mostpreferably 2.5 mass % or less.

According to the process of the present invention, the thermoplasticresin containing an electrical-chargeability enhancing agent can bemelted and extruded in the form of fibers from the melt-extrudingapparatus. The melt-extrusion may be carried out by simple extruding ofthe thermoplastic resin fibers 20 from the melt-extruding apparatus 10as shown in FIG. 1. Alternatively, a heated gas may be blown onto, or anelectric field may be applied to, the melt-extruded thermoplastic resinfibers after extrusion from the melt-extruding apparatus 10. The blowingof the heated gas, or the application of the electric field, ispreferably carried out before passing the fibers through the mist space.

When the fibers are simply extruded from the melt-extruding apparatus, aconventional die such as that used in a spun-bonding method may be used.When a heated gas is blown after extrusion from the melt-extrudingapparatus, a conventional die such as that used in a melt-blowing methodmay be used. When an electric field may be applied after extrusion fromthe melt-extruding apparatus, a conventional die such as that used in anelectrostatically extruding method may be used. In these methods, themethod of blowing a heated gas is preferable, because the fibers aredrawn and fined, and thus many properties are improved. For example, anelectret article having a high collection efficiency, an electretarticle having an excellent feeling, an electret article having anexcellent separating performance, or an electret article having anexcellent masking property can be produced.

The temperature of the blowing gas is higher than the melting point ofthe thermoplastic resin, and lower than a decomposition temperature ofthe thermoplastic resin. The amount of the heated blowing gas is notparticularly limited, but is preferably 5-fold to 2000-fold (mass)larger than the mass of the thermoplastic resin extruded. The gas is notparticularly limited, but preferably air is used in view of themanufacturing environment of the article.

As shown in FIG. 1, the thermoplastic resin fibers 20 melt-extruded fromthe melt-extruding apparatus 10 are electrically charged by passingthrough the mist space 30 formed from the droplets 31 of the polarliquid. When passing, an electric field may or may not be applied to themist space. Electrical charge may be imparted without applying anelectric field. The polar liquid used to form the droplets is notparticularly limited, but for example, is water, alcohol, acetone,ammonia, or the like. The polar liquid as above may be used alone, or ina combination thereof. From the point of view of the manufacturingenvironment, it is preferable to use water, particularly, only water.

In the process for manufacturing an electret article according to thepresent invention, a drying procedure can be omitted, by fining anaverage diameter of the droplets used to less than 20 μm. The reason whythe thermoplastic resin fibers are not wetted is unclear, when theaverage diameter of the droplets is less than 20 μm. The presentinventors believe that smaller droplets have a higher surface tension,and thus, do not wet the thermoplastic resin fibers when in contacttherewith. Therefore, it is supposed that the tendency not to wet thethermoplastic resin fibers would be strengthened as the surface tensionis increased. Therefore, the average diameter of the droplets ispreferably 15 μm or less, more preferably 12 μm or less. Further, it issupposed that if the average diameter of the droplet becomes smaller inthe mist space formed from a given mass of the droplets, the number ofdroplets acting on the thermoplastic resin fibers is increased, andthus, the amount of electrical charge can be further increased. There isno lower limit of the average diameter of the droplets, but the averagediameter is suitably about 0.1 μm. When the average diameter is smallerthan 0.1 μm, the amount of electrical charge is liable to be lowered.The term “average diameter of the droplets” as used herein means aSauter's mean diameter obtained by a measure using a laser Dopplerparticle size distribution measuring equipment.

The mist space used in the present invention may be formed from a numberof droplets floating or hanging as a mass in a spatial area; preferablya mist space formed from a number of droplets moving in a givendirection or in various directions; or a mist space formed from floatingand moving droplets. The mist space may be formed by a commerciallyavailable droplets-spraying apparatus, such as an atomizer of “AKIMist”(tradename; Ikeuchi Co., Ltd.) or “AKIJet” (tradename; Ikeuchi Co.,Ltd.).

In the process of the present invention, the thermoplastic resin fibersmay be passed through the mist space in any direction. However, it ispreferable that the thermoplastic resin fibers are passed through themist space in a direction perpendicular to the spraying direction of thedroplets from the droplets-spraying apparatus, because the amount ofelectrical charge can be increased.

In the process of the present invention, the resin-droplet percentage ofthe formula:(Wp/Wf)×100is preferably 500 or more. When the resin-droplet percentage is 500 ormore, the amount of electrical charge can be increased. It is believedthat this is because the number of the droplets acting on thethermoplastic resin fibers is increased. Further, as mentioned above, ifthe average diameter of the droplet becomes smaller in the mist spaceformed from a given mass of the droplets, the number of the dropletsacting on the thermoplastic resin fibers is increased, and thus, theamount of electrical charge can be increased. The effect of increasingthe amount of electrical charge becomes more excellent, as the value ofthe resin-droplet percentage is increased, and thus, the resin-dropletpercentage is more preferably 600 or more, particularly preferably 700or more, most preferably 1000 or more. There is no upper limit of theresin-droplet percentage, but is preferably a value at which thethermoplastic resin fibers are not wetted. Such an upper limit of theresin-droplet percentage at which the thermoplastic resin fibers are notwetted may be experimentally determined, if necessary.

The resin-droplet percentage can be suitably obtained by adjusting theamount of the fibers extruded from the melt-extruding apparatus 10 andthe amount of the droplets sprayed from the droplets-spraying apparatus40, as shown in FIG. 1. The number of the droplets-spraying apparatusused to form the mist space is not particularly limited. For example,two droplets-spraying apparatuses 40, 41 may be placed as shown in FIG.1, or one apparatus or three or more apparatuses may be used. When twoor more droplets-spraying apparatuses are used, various conditions, suchas the average diameter of the droplets, the amount of the droplets, thevelocity of the droplets, or properties or kinds of the polar liquid maybe applied.

In the process of the present invention, as shown in FIG. 1, theelectret article 60 can be manufactured by passing the thermoplasticresin fibers 20 through the mist space 30 as shown above to electricallycharge the fibers 20, and collecting the electrically chargedthermoplastic resin fibers 21 on a suitable support, such as a beltconveyer 50. The resulting electret article 60 is not wetted, andtherefore, it is possible to omit a drying procedure after thethermoplastic resin fibers 20 are passed through the mist space, in thepresent invention. The expression “the drying procedure is notnecessary” as used herein means that, for example in the embodiment asshown in FIG. 1, a procedure for drying an electret (i.e., electricallycharged) thermoplastic resin fibers 21 before collecting or an electret(i.e., electrically charged) article 60 after collecting is notnecessary. Further, for example, in the embodiment wherein the heatedgas is blown onto the thermoplastic resin fibers 20 from themelt-blowing apparatus, the above expression means that a dryingprocedure is not necessary after the thermoplastic resin fibers arepassed through the mist space. The unnecessity of the drying step maybring about additional advantageous effects. For example, if anelectrically charged article is dried by heating to enhanceproductivity, the amount of electrical charge may be lowered.Nevertheless, the present invention does not have such a defect.

The collecting means are not particularly limited. For example, the beltconveyer 50 as shown in FIG. 1, or a roller may be used. Further, thecollecting means such as the belt conveyer or the roller used may beporous or nonporous. When a porous collecting means such as the beltconveyer or the roller is used, a suction apparatus may be placed underthe collecting means such as the belt conveyer or the roller to suck theelectret (or electrically charged) thermoplastic resin fibers andprevent disturbance of the electret article. The surface of thecollecting means such as the belt conveyer or the roller may be a planeor curved, or alternatively have other three-dimensional shapes. Whenthe collecting means such as the belt conveyer or the roller has such athree-dimensional surface, the electrically charged thermoplastic resinfibers are collected in the form of the three-dimensional shape, andthus a forming procedure may be omitted after the collection. Further,for example, a porous or nonporous material may be located on thecollecting means such as the belt conveyer or the roller, and it can beeasily integrated with the electret article. An example of the porousmaterial is a sheet such as a woven fabric, knitted fabric, nonwovenfabric, or a complex thereof, a porous film such as a perforated film, afoam, or a complex thereof. An example of the nonporous material is anonporous film.

A shape, a form, or a condition of the electret article varies withthose of the thermoplastic resin fibers. The electret article may be afibrous aggregate (or web) or a film. When the electret article is usedas a gas filter, which is a preferred embodiment of the presentinvention, the electret article is preferably a fibrous aggregate, morepreferably a melt-blown fibers aggregate or web formed from fine fibersand having an excellent collection efficiency.

The electret article manufactured by the process according to thepresent invention contains a large amount of electrical charge, and thusmay be used, for example, as a gas filter for air or the like, a liquidfilter for oil, water, or the like, a filter for a mask such as a moldmask, a medical material for recovering or improving healthy, a wipingmaterial, dust-proof clothing, a sensor element of a sonic wave orvibration, or in other applications.

The electret article manufactured by the process according to thepresent invention may be used in the various applications as above, andsubsequent processing may be carried out depending on the applications.For example, when the electret article is used as a gas filter, which isa preferred embodiment of the present invention, the electret article ispreferably pleated to broaden a filterable area thereof.

EXAMPLES

The present invention now will be further illustrated by, but is by nomeans limited to, the following Examples.

Example 1

A starting resin was a polypropylene pellet containing 1 mass %poly[{(6-(1,1,3,3-tetramethylbutyl)imino-1,3,5-triazine-2,4-diyl){(2,2,6,6-tetramethyl-4-piperidyl)imino}hexamethylene{(2,2,6,6-tetramethyl-4-piperidyl)imino}}],[a hindered amine based electrical-chargeability enhancing agent:CHIMASSORB 944FD (trade name); Chiba Specialty Chemicals Co., Ltd.] withrespect to a total mass of the resin and the agent. The volume specificresistivity of the polypropylene was higher than 10¹⁴ Ω·cm. Thepolypropylene pellet was melt-blown by a usual melt-blowing apparatuswhile an air having a temperature of 300° C. was blown onto the fibers,to obtain melt-blown fibers. A mass of the air blown was 1500 times(mass) the mass of polypropylene.

The resulting melt-blown fibers were passed through a mist space toobtain electret melt-blown fibers while being spun from the melt-blowingapparatus. The mist space was formed from droplets sprayed from adroplet-spraying apparatus [AKIMist (trade name); Ikeuchi Co., Ltd.].The average diameter of the droplets sprayed from the droplet-sprayingapparatus was 10 μm. The mist space was formed at a downstream side fromthe position where the heated air was blown. The mist space had aresin-droplet percentage of 1150, and a variation coefficient of 0.05.

The electret melt-blown fibers were collected on a belt conveyer made ofa stainless steel mesh without a drying step while being sucked from theopposite side of the belt conveyer, to thereby obtain electretmelt-blown fibers web having an average fiber diameter of 5 μm, i.e., anelectret article.

Example 2

A starting resin was a polypropylene pellet containing 3 mass % hinderedamine based electrical-chargeability enhancing agent as used in Example1, with respect to a total mass of the resin and theelectrical-chargeability enhancing agent. The volume specificresistivity of the polypropylene was higher than 10¹⁴ Ω·cm. Thepolypropylene pellet was melt-blown by a usual melt-blowing apparatuswhile an air having a temperature of 260° C. was blown onto the fibers,to obtain melt-blown fibers. A mass of the air blown was 50 times (mass)the mass of the polypropylene.

The resulting melt-blown fibers were passed through a mist space toobtain electret melt-blown fibers while being spun from the melt-blowingapparatus. The mist space was formed from droplets sprayed from thedroplet-spraying apparatus used in Example 1. An average diameter of thedroplets sprayed from the droplet-spraying apparatus was 10 μm. The mistspace was formed at a downstream side from the position where the heatedair was blown. The mist space had a resin-droplet percentage of 750, anda variation coefficient of 0.05.

The electret melt-blown fibers were collected on a belt conveyer made ofa stainless steel mesh without a drying step while being sucked from theopposite side of the belt conveyer, to thereby obtain electretmelt-blown fibers web having an average fiber diameter of 7 μm, i.e., anelectret article.

Comparative Example 1

The procedures of Example 1 were repeated except that an averagediameter of the droplets sprayed from a droplet-spraying apparatus was30 μm. However, the melt-blown fibers collected on the conveyer werethoroughly saturated with water, and a drying step was required.

Comparative Example 2

The procedures of Example 1 were repeated except that a polypropylenepellet without a hindered amine was used, to obtain an electret articlewithout a drying step.

Comparative Example 3

The procedures of Example 1 were repeated except that a polypropylenepellet without a hindered amine was used, and the fibers were not passedthrough the mist space, to obtain a web of the melt-blown fibers withouta drying step.

Evaluation of Properties

(1) Measurement of Collection Efficiency

An efficiency to collect atmospheric airborne duct having a diameter of0.3 to 0.5 μm by each of the electret articles of Examples 1 and 2 andComparative Example 2 and the melt-blown fibers web of ComparativeExample 3 was measured, using air at an air velocity of 10 cm/second.The results are listed in Table 1. The melt-blown fibers web ofComparative Example 1 were thoroughly saturated when collected, andthus, a drying step was required. Therefore, a collection efficiencythereof could not be measured.

(2) Measurement of Pressure Loss

A pressure loss of each of the electret articles of Examples 1 and 2 andComparative Example 2 and the melt-blown fibers web of ComparativeExample 3 was measured, using an atmospheric air at an air velocity of10 cm/second. The results are listed in Table 1. The melt-blown fibersweb of Comparative Example 1 were thoroughly saturated when collected,and thus, a drying step was required. Therefore, a pressure loss thereofcould not be measured.

(3) Calculation of γ Value

A penetration percentage (P; unit=%) was calculated from the collectionefficiency (E; unit=%) obtained as above by an equation: P=100−E. Then,a γ value (unit=1/Pa) was calculated from the resulting penetrationpercentage (P; unit=%) and the pressure loss (AP; unit=Pa) obtained asabove by an equation:γ=[−ln(P/100)]/ΔP.

The results are listed in Table 1. An electret article having a larger γvalue has a smaller pressure loss and thus a smaller penetrationpercentage. Therefore, an electret article having a large γ value can bepreferably used as an excellent filter. TABLE 1 Mass per CollectionPressure unit area Thickness efficiency loss γ (g/m²) (mm) (%) (Pa)(1/Pa) Example 1 44 0.98 99.79 26 0.237 Example 2 20 0.3 63.0 3.6 0.276Comparative Immeasurable because of saturated condition of the Example 1melt-blown fibers aggregate Comparative 46 0.76 31.88 34 0.011 Example 2Comparative 45 0.86 25.45 32 0.009 Example 3

As apparent from Table 1, the electret articles produced by the processof the present invention had an excellent collection efficiency, despitea lower pressure loss in comparison with that of the melt-blown fibersweb of Comparative Examples 2 and 3. Therefore, it is assumed thatelectret articles produced by the process of the present invention had alarge amount of electrical charge.

INDUSTRIAL APPLICABILITY

According to the present invention, an electret article having a largeamount of electrical charge can be produced by a simple process withouta drying step.

When the resin-droplet percentage of the formula (Wp/Wf)×100 is 500 ormore in the process of the present invention, the amount of electricalcharge can be increased.

If the heated gas is blown onto the melt-extruded thermoplastic resinfibers in the process of the present invention, the fibers are drawn andbecome fine: Thus, an electret article formed from fine fibers andhaving enhanced properties can be obtained.

When the volume specific resistivity of the thermoplastic resin is 10¹⁴Ω·cm or higher, particularly 10¹⁶ Ω·cm or higher, in the process of thepresent invention, the amount of electrical charge can be increased.

When the polar liquid is water in the process of the present invention,the environment wherein the electret article is produced becomes good.

When the electrical-chargeability enhancing agent is at least onecompound selected from a group consisting of a hindered amine compound,a metallic salt of a fatty acid, a metallic oxide and an unsaturatedcarboxylic acid-modified high-molecular compound, in the process of thepresent invention, the amount of electrical charge can be remarkablyincreased.

When the average diameter of the droplet is 15 μm or less in the processof the present invention, the amount of electrical charge can be furtherincreased. This is because the number of droplets can be increased underthe condition of same total amount of droplets.

According to the apparatus of the present invention, an electret articlehaving a large amount of electrical charge can be produced by a simpleapparatus without a drying means.

Although the present invention has been described with reference tospecific embodiments, various changes and modifications obvious to thoseskilled in the art are possible without departing from the scope of theappended claims.

1. A process for manufacturing an electret article, comprising passingmelt-extruded thermoplastic resin fibers through a mist spacesubstantially formed from droplets of a polar liquid, and thencollecting the fibers, wherein said thermoplastic resin fibers containelectrical-chargeability enhancing agents, and the average diameter ofsaid droplets is less than 20 μm.
 2. The process according to claim 1,wherein the thermoplastic resin fibers are not subjected to a dryingstep after passing through said mist space.
 3. The process according toclaim 1, wherein a resin-droplet percentage of the formula:(Wp/Wf)×100 wherein Wp denotes the amount of said droplets forming saidmist space and sprayed to a unit volume thereof within a certain periodof time, and Wf denotes the amount of said melt-extruded thermoplasticresin passed through said mist space within a certain period of time is500 or more.
 4. The process according to claim 1, wherein a heated gasis blown onto said melt-extruded thermoplastic resin fibers.
 5. Theprocess according to claim 1, wherein a volume specific resistivity ofsaid thermoplastic resin is 10¹⁴ ∩·cm or higher.
 6. The processaccording to claim 5, wherein a volume specific resistivity of saidthermoplastic resin is 10¹⁶ Ω·cm or higher.
 7. The process according toclaim 1, wherein said polar liquid is water.
 8. The process according toclaim 1, wherein said electrical-chargeability enhancing agent is atleast one compound selected from a group consisting of a hindered aminecompound, a metallic salt of a fatty acid, a metallic oxide, and anunsaturated carboxylic acid-modified high-molecular compound.
 9. Theprocess according to claim 1, wherein the average diameter of saiddroplets is 15 μm or less.
 10. An apparatus for manufacturing anelectric article, comprising (1) a means for melt-extruding athermoplastic resin containing electrical-chargeability enhancing agentsto form thermoplastic resin fibers; (2) a means for spraying dropletsconsisting essentially of a polar liquid to a space downstream of adirection of said thermoplastic resin extruded from said means formelt-extruding a thermoplastic resin, to thereby form a mist space, theaverage diameter of said droplets being less than 20 μm; and (3) a meansfor collecting said thermoplastic resin fibers which have been passedthrough said mist space.